Tectonic evolution from oceanic subduction to continental collision during the closure of Paleotethyan ocean: Geochronological and geochemical constraints from metamorphic rocks in the Hong'an orogen

A combined study of whole-rock major-trace elements and Sr–Nd isotopes as well as zircon U–Pb ages, trace elements and Lu–Hf isotopes was carried out for high-pressure (HP) and ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks in the Hong'an orogen, China. The results provide insights into the tectonic evolution from oceanic subduction to continental collision during the closure of Paleotethyan ocean between the South and North China Blocks. Based on the whole-rock geochemistry, eclogites in the Hong'an orogen are categorized into continental- and oceanic-type, respectively. The continental-type eclogites widely occur in the whole orogen, exhibiting general enrichments of LILE and LREE but depletion of HFSE and HREE. Zircon U–Pb dating yields protolith ages of about 750 to 1200Ma, demonstrating a tectonic affinity to the South China Block. They have both positive and negative εNd(t) and εHf(t) values for whole-rock and zircons, suggesting that their protoliths were originated from both juvenile and ancient crustal rocks. The oceanic-type eclogites only occur in the northwestern edge of the orogen, exhibiting mid-ocean ridge basalt (MORB)-like flat REE patterns and arc-like trace element patterns. Zircon U–Pb dating on the relict zircon cores of magmatic origin yields protolith ages of ~420Ma. They have variable εNd(t) and positive εHf(t) values and slightly high (87Sr/86Sr)i ratios. These geochemical features suggest that their protoliths are equivalent to backarc basin basalts (BABB) in a continental margin.